1. Field of the Invention
The present invention relates to a plasma generation electrode, a plasma reactor, and an exhaust gas cleaning apparatus. In particular, it relates to a plasma generation electrode capable of subjecting a plurality of predetermined components contained in a fluid to be treated to their respective reaction treatments with plasmas having intensities optimized on a reaction basis, by passing merely once the fluid to be treated through spaces, in which plasmas are generated, as well as a plasma reactor, and an exhaust gas cleaning apparatus.
2. Description of the Related Art
It is known that silent discharge is generated by placing a dielectric material between two electrodes and applying a high-voltage alternating current or a periodic pulse voltage, and in the plasma field resulting therefrom, active species, radicals, and ions are generated so as to facilitate reaction and decomposition of a gas. Furthermore, it is known that this can be used for removing hazardous components contained in engine exhaust gases and various incinerator exhaust gases.
For example, plasma reactors and the like have been disclosed, in which engine exhaust gases and various incinerator exhaust gases are allowed to pass through plasma fields and, thereby, NOX, carbon fine particles, HC, CO, and the like contained in the engine exhaust gases and various incinerator exhaust gases are treated (refer to Japanese Patent Application Laid-Open No. 2001-164925, for example)
However, since the magnitude of discharge voltage suitable for the treatment with the plasma is different between the above-described NOX, carbon fine particles, and the like, when these components in an exhaust gas are treated, a plurality of plasma reactors must be used independently, or a plasma must be generated in accordance with the condition for the largest discharge voltage. There are problems in that if a plurality of plasma reactors are used, the installation cost is increased, and if the discharge voltage is set at a large value, the energy loss is increased.
On the other hand, plasma reactors and the like have been disclosed, in which a plurality of component gases in a fluid to be treated, e.g., an engine exhaust gas, can be efficiently treated with one plasma reactor by using a plurality of plasmas having different intensities optimized on a reaction basis (refer to International Patent Publication WO 2005/001249 Pamphlet, for example).
As shown in FIG. 12(a) and FIG. 12(b), for example, the invention described in International Patent Publication WO 2005/001249 relates to a plasma generation electrode 201 including a plurality of unit electrodes 202 hierarchically layered at a predetermined spacing, a space V, in which both ends in one direction (gas passing direction) P are opened and both ends of the other direction (closing direction) Q are closed, is disposed between the unit electrodes 202, and a plasma can be generated in the space V by application of a voltage between the unit electrodes 202. In this plasma generation electrode 201, the unit electrode 202 is composed of a tabular ceramic material 203 serving as a dielectric material and an electrically conductive film 204 disposed in the inside of the ceramic material 203 and, in addition, the unit electrodes 202 include deficient unit electrodes 202b having a portion, in which no electrically conductive film 204 is present, somewhere in between one end and the other end in one direction P and normal unit electrodes 202a not having a portion in which no electrically conductive film 204 is present. Furthermore, the spaces V include a plurality of normal spaces Va disposed between the normal unit electrode 202a and the deficient unit electrode 202b facing each other or between the deficient unit electrodes 202b facing each other in such a way that the distance between the electrically conductive films 204 becomes equal to the distance between the unit electrodes 202 and a plurality of deficient spaces Vb disposed between the normal unit electrodes 202a facing each other with a deficient portion in the deficient unit electrode therebetween in such a way that the distance between the electrically conductive films 204 becomes larger than the distance between the electrically conductive films 204 in the normal space Va. In this manner, since the distance between the electrically conductive films 204 constituting the unit electrodes 202 for generating a plasma in the normal space Va is different from that in the deficient space Vb, the intensity of the plasma generated in the normal space Va is allowed to become different from that in the deficient space Vb. Consequently, it becomes possible to efficiently treat a plurality of predetermined components, which are contained in the fluid to be treated, with plasmas having intensities optimized on a reaction basis, by passing merely once the fluid to be treated. Here, FIG. 12(a) and FIG. 12(b) schematically show a known plasma generation electrode. FIG. 12(a) is a sectional view of the section cut along a plane perpendicular to one direction (gas passing direction). FIG. 12(b) is a sectional view of the section taken along a line B-B′ shown in FIG. 12(a).
As described above, according to the plasma generation electrode 201 shown in FIG. 12(a) and FIG. 12(b), since the distance between the electrically conductive films 204 constituting the unit electrodes 202 for generating a plasma in the normal space Va is different from that in the deficient space Vb, the intensity of the plasma generated in the normal space Va is allowed to become different from that in the deficient space Vb. However, there is a special relationship between the distance W1 between the unit electrodes 202 in the normal space Va and the distance W2 between the unit electrodes 202 in the deficient space Vb and, therefore, it is not always easy to determine each value independently. Consequently, the individual gas components can be subjected to reaction treatments with approximately suitable plasma intensities, but it is not always easy to subject the individual gas components to reaction treatments with optimized plasma intensities. Here, the above-described special relationship between the distance W1 and the distance W2 is represented by “W2=W1×α+T×(α-1), where α is a natural number and T is the thickness of the unit electrode”. This refers to that since the deficient space Vb is disposed by dropout of a part of the unit electrode 202, the distance W2 must take on a limited value, the sum of W1 multiplied by a natural number and T multiplied by (a natural number −1), relative to the distance W1 and the thickness T of the unit electrode.
Since the plasma generation electrode must be supported in such a way that a plurality of types of distances between the unit electrodes are ensured, the support structure tends to become complicated.
As shown in FIG. 13(a) and FIG. 13(b), for example, International Patent Publication WO 2005/001249 discloses a plasma generation electrode 211, in which unit electrodes 212 include normal unit electrodes 212a and deficient unit electrodes 212b, and the deficient unit electrode 212b is formed by dropout of merely a part of an electrically conductive film 204 constituting the unit electrode 212. Here, FIG. 13(a) and FIG. 13(b) schematically show a known plasma generation electrode. FIG. 13(a) is a sectional view of the section cut along a plane perpendicular to one direction (gas passing direction). FIG. 13(b) is a sectional view of the section taken along a line C-C′ shown in FIG. 13(a). In this aspect, a ceramic material 213 has no deficient portion. Consequently, the support structure becomes simple, and this is favorable from the view point of the support of plasma generation electrode. However, the relationship between the distance W3 between the unit electrodes 212 in the normal space Va and the distance W4 between the unit electrodes 212 in the deficient space Vb is similar to the above-described special relationship between the distance W1 and the distance W2. Therefore, likewise, it is not always easy to determine the intensity of plasma generated in the normal space Va and that in the deficient space Vb independently.